The present invention relates to sheathed heaters, and more particularly to a filler used as an insulator between a heating wire and a metal outer tube or sheath of a heater for domestic and industrial applications.
It is well known in the prior art that sintered magnesia and electro-fused magnesia are used as an electrically insulating filler for sheathed heaters. Such magnesia products are normally available in the form of particles, and the shapes of such particles are nonglobular, such as prisms and scales. It has been recognized that the use of such materials has the following inherent disadvantages:
(i) In the manufacture of the sheathed heater, it takes a very long time to pack the magnesia particles into the sheath due to their low fluidity, resulting in a low production efficiency. PA0 (ii) It is difficult to achieve a high packing density. PA0 (iii) The inner wall of outer metal sheath and the interior heating wire are frequently damaged by corners and edges of the particles during packing and/or swaging of the sheath. PA0 (iv) Reduction of the particle size in the sheath during swaging of the sheath is not uniform, resulting in uneven electric properties.